Book holder adapter



FIG. 1 is a front, top and side perspective view of the book holder adapter embodying the design of the present invention;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a side elevational view thereof, the side opposite being a mirror image of the side shown in FIG. 2; and,

FIG. 4 is a bottom plan view thereof. 

The ornamental design for a book holder adapter, as shown and described. 